Cyclone separator

ABSTRACT

This invention relates generally to cyclone separators. The separator of the present invention may find application in removing a lighter phase from a large volume of a denser phase, such as oil from water, with minimum contamination of the more voluminous phase. Generally, cyclone separators are designed for the opposite purpose, that is, removing a denser phase from a large volume of a lighter phase, with minimum contamination of the less voluminous phase. In one form of the present invention, a typical starting liquid-liquid dispersion would contain under 1% by volume of the lighter (less dense) phase, but it could be more.

According to one aspect of the present invention there is provided acyclone separator comprising at least a primary portion having generallythe form of a volume of revolution having a first end and a second endthe diameter at said second end being less than the diameter at saidfirst end, a single inlet with at least a tangential component at oradjacent said first end for introducing feed to be separated into thecyclone separator and the separator further including at least twooutlets.

In one form of the cyclone separator of the invention the followingrelationship applies: wherein d_(i) is the diameter of the primaryportion at the inlet, d₂ is the diameter of the second end of the saidprimary portion, d_(i) is greater than d₂ and is twice the radius fromthe cyclone separator axis to the mean point when flow enters thecyclone separator and is greater than d_(i), and A_(i) is the area ofthe inlet where flow enters the cyclone separator measured in the planeincluding the cyclone axis and the said mean point of flow entry then:##EQU1## is from 3 to 12 and preferably from 4 to 10 and more preferablyfrom 6 to 8. The above term will be termed the "swirl coefficient" andis discussed in more detail later.

The cyclone separator may further include a generally axiallysymmetrical secondary portion at the aforementioned second end of andsubstantially coaxial with the primary portion. In another form theseparator may further include a tertiary portion at the end of andsubstantially coaxial with said secondary portion remote from theprimary portion. It will be appreciated that the separator could alsoinclude additional portions to that described above.

In one particular form of the invention wherein d₂ which is the diameterof the primary portion at the aforementioned second end measured at apoint z₂ where the condition first applies that: ##EQU2## for all z>z₂where z is the distance along the cyclone separator axis downstream ofthe inlet.

Preferably the inlet gives into an inward spiralling feed channel whichmay be involute in form. In one form the feed channel subtends at least360° at the cyclone axis. The feed channel may also converge bysubstantially equal radial decrements per unit angle around the axis.The inlet may enter the cyclone with a component in the axial downstreamdirection.

In another form of the invention, the cyclone separator is defined asfollows. The cyclone separator has a primary portion having generallythe form of a volume of revolution with a single inlet (preferablytangential, and preferably with an inwards spiralling feed channel suchas an involute entry) for introducing feed to be separated into thecyclone separator and, adjacent to the primary portion and substantiallycoaxial therewith, a generally axially symmetrical secondary portionconverging (preferably uninterruptedly) into a tertiary portion. Theprimary portion may have an axial overflow outlet opposite the secondaryportion (i.e. in its end wall). In the cyclone separator, the followingrelationships (i)-(v) apply: where d₁ is the diameter of the cyclone inthe primary portion where flow enters (but neglecting any feed channel),d_(i) is twice the radius at which flow enters the cyclone (i.e. twicethe minimum distance of the tangential component of inlet centrelinefrom the axis), A_(i) is the cross-sectional area of the inlet at entryto the cyclone in a plane parallel to the axis of the cyclone andperpendicular to the component of the inlet centreline not parallel tothe cyclone axis, d₂ is the diameter of the cyclone where the primaryportion joins the secondary portion the point of junction being definedas being at the axial positions z₂ (measured away from the inlet plane)where the condition first applies that: ##EQU3## for all z>z₂ where d isthe cyclone diameter at z,

d₃ is the cyclone diameter where the secondary portion joins thetertiary section and is defined as the diameter at z₃ where d/d₃ >0.98for all z>z₃, d_(o) is the minimum internal diameter of the axialoverflow outlet, then: ##EQU4## where α is the half angle of convergenceof the secondary portion i.e. ##EQU5##

The feed channel may be fed from a duct directed substantiallytangentially into the primary portion, the (outer) wall of the channelconverging to the principal diameter of the primary portion d₁, forexample by sustantially equal radial decrements per unit angle aroundthe axis, preferably attaining the diameter d₁ after 360° around theaxis.

The feed channel need not be in a plane normal to the axis, and ifoffset, so as to adopt a generally helical form, may attain the diameterd₁ after more than 360° (e.g. 720°) around the axis. Using a singleinlet, only a single feed connection has to be made to the cycloneseparator, which is simpler to install and saves space, importantadvantages on board a ship or oil-rig; this arrangement also makes formanufacturing simplicity. The expression ##EQU6## which as mentionedabove is termed the `swirl coefficient` S, is a reasonable predictor ofthe ratio of velocities tangentially:axially of flow which has enteredthe cyclone and which has reached the plane of d₂ and, (with a dispersedlighter phase, as is of particular interest in order to be able tocreate an internal flow structure favourable for separation at a lowsplit ratio* of the order of 1% then the half-angle of convergenceaveraged over the whole secondary portion is 20' to 2°, preferably lessthan 1°, more preferably less than 52', preferably at least 30'. S isfrom 3 to 12, preferably from 4 to 10, more preferably from 6 to 8. Theconvergence averaged from the diameter d₁ measured in the inlet plane tothe diameter d₂ may be the fastest (largest cone half-angle) in thecyclone, and may be from 5° to 45°. (The inlet plane is that planenormal to the cyclone axis including the centroid of the area A_(i)).The primary portion should be such that the angular momentum of materialentering from the inlet is substantially conserved into the secondaryportion.

Preferably, d₃ /d₂ is less than 0.75 (more preferably less than 0.7) andpreferably exceeds 0.25 (more preferably exceeding 0.3). Preferablywhere the internal length of the downstream tertiary portion is l₃, l₃/d₃ is at least 1, more preferably at least 5; it is typically about 10and may be as large as desired, such as at least 40. For space reasonsit may be desired to curve the dense-phase-outlet portion gently, and aradius of curvature of the order of 50d₃ is possible, and gentlecurvature of the cyclone axis is feasible. d₁ /d₂ may be from 11/4 to 3.Preferably d_(o) /d₂ is at most 0.15 and preferably at least 0.008,possibly from 0.01 to 0.1, such as 0.02 to 0.06. Pressure drop in theaxial overflow outlet should not be excessive, and therefore the lengthof the "d_(o) " portion of the axial overflow outlet should be kept low.The axial overflow outlet may reach its "d_(o) " diameterinstantaneously or by any form of abrupt or smooth transition, and maywiden thereafter by a taper or step. The axial distance from the inletplane to the "d_(o) " point is preferably less than 4d₂. The actualmagnitude of d₂ is a matter of choice for operating and engineeringconvenience, and may for example be 10 to 100 mm.

In another version, according to the invention, at least part of thegenerator of the primary portion or of the separation portion or of bothis curved. The generator may be, for example, (i) a monotonic curve(having no points of inflexion) steepest at the inlet-portion end andtending to a cone-angle of zero at its open end, or (ii) a curve withone or more points of inflexion but overall converging towards thedownstream tertiary portion, preferably never diverging towards thedownstream tertiary portion.

The invention extends to a method of removing a lighter phase from alarger volume of denser phase, comprising applying the phases to thefeed of a cyclone separator as set forth above, the phases being at ahigher pressure than in the axial overflow outlet and in the downstreamend of the downstream tertiary portion; in practice, it will generallybe found that the pressure out of the downstream outlet portion willexceed that out of the axial overflow outlet.

This method is particularly envisaged for removing up to 1 part byvolume of oil (light phase) from over 19 parts of water (denser phase),(such as over 99 parts), such as oil-field production water or sea waterwhich may have become contaiminated with oil, as a result of spillage,shipwreck, oil-rig blow out or routine operations such as bilge-rinsingor oil-rig drilling. The ratio of flow rates upstream outlet/downstreamoutlet (and hence the split ratio) has a minimum value for successfulseparation of the oil, which value is determined by the geometry of thecyclone (especially by the value of d_(o) /d₂) but preferably thecyclone is operated above this minimum value, e.g., by back pressure forexample provided by valving or flow restriction ouside the definedcyclone. Thus preferably the method comprises arranging the split ratioto exceed 11/2(d_(o) /d₂)², preferably to exceed 2(d_(o) /d₂)².

The method may further comprise, as a preliminary step, eliminating freegas from the phases such that in the inlet material the volume of anygas is not more than 10%, such as not more than 1/2%.

As liquids normally become less viscous when warm, water for examplebeing approximately half as viscous at 50° C. as at 20° C., the methodis advantageously performed at as high a temperature as convenient. Theinvention extends to the products of the method (such as concentratedoil, or cleaned water).

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 shows, schematically, a cross-section taken on the axis of acyclone separator according to the invention, and

FIG. 2 is a view down the axis of the cyclone separator. The drawingsare not to scale.

A primary portion 1 having generally the shape of a volume of revolutionhas a spiral feed channel 8 which in one form may be involute fed from aduct 9 directed tangentially into the widest part of the primaryportion 1. The width (radially) of the duct 9 is r_(v) (max), and thechannel 8 converges smoothly to the principal diameter d₁ of the inletportion; thus r_(v) diminishes linearly to zero at 360° after the pointof entry of the duct 9 into the cyclone separator. This is best seen inFIG. 2, which is a view down the axis of the cyclone separator, whoseend wall 11 has been removed. Coaxial with the primary portion 1, andadjacent to it, is a secondary portion 2, which opens at its far endinto a coaxial generally cyclindrical downstream tertiary portion 3. Theportion 3 opens into collection ducting 4. The channel 8 alternativelymay be slightly angled towards the secondary portion 2 to impart anaxial component of velocity, and in that case may be helical, reducingto the principal diameter d₁ after say 2 full revolutions.

The primary portion 1 has an axial overflow outlet 10 opposite thesecondary portion 2.

In the present cyclone separator, the actual relationships are asfollows:

    d.sub.1 /d.sub.2 =1.5

The half-angle of conicity of the secondary portion 2=40' (T₂ onFigure).

The average half-angle of conicity of the primary portion 1=10° (T₁ onFigure).

Where the axial extent of the duct 9 is l_(i), l_(i) /d_(l) =1/2 (moreprecisely 30/57).

    l.sub.3 /d.sub.3 =40

    d.sub.o d.sub.2 =0.04

This cyclone should accordingly be operated at a split ratio ##EQU7## ofmore than 11/2(0.04)², i.e. more than 0.24%.

To the principal diameter d₁ of the primary portion, there must be addeda radial amount r_(v) decreasing smoothly from 91/2 mm (maximum) tozero, for the volute inlet. ##EQU8## noting that l_(i) r_(v)(max)=A_(i), as defined previously and (d₁ +r_(v) (max))=d₁ as definedpreviously, the foregoing expression thus being the swirl coefficient Sof this cyclone. The taper which averages out as T₁ actually curves overa radiussing R₁ (radius=5 mm) into a frustoconical part of the primaryportion 1.

d₂ =38 mm. This is regarded as the `cyclone diameter` and for manypurposes can be anywhere within the range 10-100 mm, for example 15-60mm; with excessively large d₂, the energy consumption becomes large tomaintain effective separation while with too small d₂ unfavourableReynolds Number effects and excessive shear stresses arise.

The cyclone separator can be in any orientation with insignificanteffect.

The wall 11 is smooth as, in general, irregularities upset the desiredflow patterns, within the cyclone. For the best performance, all otherinternal surfaces of the cyclone should also be smooth. However, wall 11need not be plane; it could be dished (convex or concave) or may have asmall upstanding circular ridge concentric with the outlet 10 to assistthe flow moving radially inward near the wall, and the outer `fringe` ofthe vortex, to recirculate in a generally downstream direction forresorting. The outlet 10 is a cylindrical bore as shown, but its minimumdiameter d_(o) could instead be approached by a smooth curve of the wall11, and the outlet 10 could thereafter diverge. Where the minimumdiameter is provided by an orifice plate lying flush on the wall 11 andcontaining a central hole of diameter d_(o) leading directly to arelatively large bore, the different flow characteristics appear to havea slightly detrimental, though not serious, effect on performance. Theoutlet 10 may advantageously be divergent in the direction of overflow,with the outlet widening thereafter at a cone half-angle of up to 10°.In this way, a smaller pressure drop is experienced along the outlet,which must be balanced against the tendency of the illustratedcylindrical bore (cone half angle of zero) to encourage coalescence ofdroplets of the lighter phase, according to the requirements of theuser.

To separate oil from water (still by way of example), the oil/watermixture is introduced through the feed channel 8 at a rate of 70-110l/min with any free gas in the inlet limited to 1/2% by volume. Themixture is a dispersion of 0.15 parts by volume of Forties field crudeoil in 99.85 parts of fresh water at 15.5° C. A convergence T₁ ofaverage angle to the axis 10°, made up of a radiussed portion R₁(radius=5 mm) and a frusto-conical portion, brings the inlet portiondown to the separation portion. Alternatively worded, 10° is theconicity (half-angle) of the notional average frustrum represented byT₁. The dispersion swirls into the second portion 2, conserving itsangular momentum. The bulk of the oil separates to form an axial oilcore within an axial vortex in the second portion 2.

The spiralling flow of the water plus remaining oil then enters thethird portion 3. Some remaining oil separates within a continuation ofthe axial vortex in the third portion 3. The cleaned water leavesthrough the ducting 4 which may contain any necessary flow restrictionand the cleaned water may be collected, for return to the sea forexample or fur further cleaning, for example in a second similar oridentical cyclone or a bank of cyclones in parallel.

Operating this cyclone at a split ratio of about 0.9% by the use ofvalves to control the flow out of the two outlets, oil entrained alongthe axis of the vortex moves axially to the overflow outlet and may becollected for dumping, storage or further separation, since it willstill contain some water. In this case too, the further separation mayinclude a second similar or identical cyclone.

Where the mean diameter of the oil droplets was 70 micrometers, ##EQU9##rose with flowrate from 0.955 at 70 l/min to 0.966 at 110 l/mintherefore suggesting no significant drop break-up that would be manifestas a more constant or falling efficiency. At 100 l/min efficiency wasidentical with a known twin-tangential inlet cyclone.

At a smaller drop-size mean diameter 50 micrometers and flowrate 100l/min, an efficiency of 0.922 was obtained (0.924 in the known cyclone).Other performance characteristics such as the volumetric flowratethrough the overflow 10 as a proportion of that through the ducting 4,or the pressure drop between 8 and 4, were broadly similar to the knowncyclone, i.e., that described in UK Specification No. 2102311.

The single duct 9, apart from its advantages in installation andmanufacture and space-saving, is more amenable to flow control than amultiple inlet; a simple movable plate or plug or gate adjustable withinthe duct can be provided to control the inlet flow and, by varying theeffective cross-sectional area of the inlet, to control the swirlcoefficient S.

What is claimed is:
 1. A cyclone separator for the separation of amixture including multiphase liquids, the separator comprising a primaryportion having generally the form of a volume of revolution and having afirst end and a second end, the diameter d₂ at said second end beingless than the diameter d₁ at said first end, a single inlet with atleast a tangential component at said first end of said primary portionfor introducing the mutiphase mixture to be separated into the cycloneseparator and at least two outlets; characterized in that the followingrelationship applied: where d₁ is the diameter of the cyclone in theinlet portion where flow enters (but neglecting any feed channel), d_(i)is twice the radius at which flow enters the cyclone (i.e. twice theminimum distance of the tangential component of the inlet center linefrom the axis), d_(o) is the diameter of said overflow outlet, A_(i) isthe cross-sectional area of the inlet at entry to the cyclone in a planeparallel to the axis of the cyclone and perpendicular to the componentof the inlet center line not parallel to the cyclone axis, d₂ is thediameter of the primary portion at said second end and is measured at apoint z₂ where the condition first applies that: ##EQU10## for all zgreater than z₂ where z is the distance along the cyclone separator axisdownstream of the plane containing the inlet and d is the diameter ofthe cyclone at z, then ##EQU11## is from 3 to
 12. 2. A cyclone separatoraccording to claim 1 wherein ##EQU12## is from 4 to
 10. 3. A cycloneseparator according to claim 2 wherein ##EQU13## is from 6 to
 8. 4. Acyclone separator according to claim 1, further including a generallyaxially symmetrical secondary portion at said second end andsubstantially coaxial with said primary portion and wherein d/d₃ >0.98for all z>z₃, where d₃ is the diameter of the end of the secondaryportion remote from said primary portion.
 5. A cyclone separatoraccording to claim 4 further including a tertiary portion substantiallycoaxial with said secondary portion and at the end of said secondaryportion remote from said primary portion.
 6. A cyclone separatoraccording to claim 4 wherein 20'<α<2° where α is the half angle ofconvergence of the secondary portion; i.e. ##EQU14##
 7. A cycloneseparator according to claim 6 wherein α is from 30' to 52'.
 8. Acyclone separator according to claim 4 wherein d₂ <0.9d_(i).
 9. Acyclone separator according to claim 8 wherein d₃ <0.9d₂.
 10. A cycloneseparator according to claim 9 wherein d₃ /d₂ is from 0.25 to 0.75. 11.A cyclone separator according to claim 4 wherein d₃ <0.9d₂.
 12. Acyclone separator according to claim 4 wherein the secondary portion isa volume of revolution, the generator of which is a continuously curvedline.
 13. A cyclone separator according to claim 1 wherein d₂ <0.9d_(i).14. A cyclone separator according to claim 1 wherein the inlet givesinto an inwardly spiralling feed channel.
 15. A cyclone separatoraccording to claim 1 wherein the inlet enters the cyclone with acomponent in the axial downstream direction.
 16. A cyclone separatoraccording to claim 1 wherein one of said outlets is an overflow outletat said first end of first said primary portion.
 17. A cyclone separatoraccording to claim 16 wherein d_(o) /d₂ <0.2 where d_(o) is the diameterof said one of said outlets.
 18. A cyclone separator according to claim17 wherein d_(o) /d₂ is from 0.008 to 0.1.
 19. A cyclone separatoraccording to claim 1 wherein the primary portion is a volume ofrevolution, the generator of which is not a straight line.
 20. A methodof removing a lighter phase from a larger volume of a denser phase,comprising providing a cyclone separator having at least a primaryportion having generally the form of a volume of a revolution and havinga first end and a second end, the diameter at said second end being lessthan the diameter at said first end, a single inlet with at least atangential component at or adjacent said first end for introducing feedto be separated into the cyclone separator and the separator furtherincluding at least two outlets, applying the phases to the feed of saidcyclone separator, and withdrawing a product at each of said outlets, inwhich cyclone separator the following relationship applies; where d₁ isthe diameter of the cyclone in the inlet portion where flow enters (butneglecting any feed channel), d_(i) is twice the radius at which flowenters the cyclone (i.e. twice the minimum distance of the tangentialcomponent of the inlet center line from the axis), A_(i) is thecross-sectional area of the inlet at entry to the cyclone in a planeparallel to the axis of the cyclone and perpendicular to the componentof the inlet center line not parallel to the cyclone axis, d₂ is thediameter of the primary portion at said second end and is measured at apoint z₂ where the condition first applies that: ##EQU15## for all zgreater than z₂ where z is the distance along the cyclone separator axisdownstream of the plane containing the inlet and d is the diameter ofthe cyclone at z, then ##EQU16## is from 3 to
 12. 21. A cycloneseparator for the separation of a mixture including multiphase liquids,the separator comprising a primary portion having generally the form ofa volume of revolution and having a first end and a second end, thediameter d₂ at said second end being less that the diameter d₁ at saidfirst end, a generally axially symmetrical secondary portion at saidsecond end and substantially co-axial with said primary portion, saidsecondary portion having a diameter d₃ at the end thereof remote fromsaid primary portion, a single involute inlet, with at least atangential component at said first end of said primary portion forintroducing the multiphase mixture to be separated into the cycloneseparator, an overflow outlet at said first end of said primary portionand an underflow outlet at the end of the cyclone separator or remotefrom said first end; characterized in that the following relationshipapplies: where d₁ is the diameter of the cyclone in the inlet portionwhere flow enters (but neglecting any feed channel), d_(i) is twice theradius at which flow enters the cyclone (i.e. twice the minimum distanceof the tangential component of the inlet center line from the axis),d_(o) is the diameter of said overflow outlet, A_(i) is thecross-sectional area of the inlet at entry to the cyclone in a planeparallel to the axis of the cyclone and perpendicular to the componentof the inlet center line not parallel to the cyclone axis, d₂ is thediameter of the primary portion at said second end and is measured at apoint z₂ where the condition first applies that: ##EQU17## for all zgreater than z₂ where z is the distance along the cyclone separator axisdownmstream of the plane containing the inlet and d is the diameter ofthe cyclone at z, then ##EQU18## and wherein

    2'<α< °

where α is the half angle of convergence of the secondary portion.